Adolescent substance abuse is a major public health concern. Residential treatment is one of the most commonly provided services as well as one of the most costly to address this problem. Little is known about the implementation of these programs or their fidelity to the principles of intervention models. The ultimate aim of the proposed study is to improve understanding of how residential substance abuse treatment works for adolescents by applying a multidimensional method to measure residential treatment fidelity. The study will evaluate treatment fidelity for approximately 45 subjects enrolled in a residential treatment center that represents one condition in a large randomized trial comparing family-based treatment to residential treatment for adolescent substance abuse. Subjects will have completed approximately 10 weeks of residential treatment, on average. The primary aims of the study are to (a) evaluate degree of program fidelity, (b) evaluate variability in program fidelity, and (c) assess the relation between adherence to program service parameters and maintenance of therapeutic milieu. The residential program that is the focus of the study incorporates two main treatment components whose implementation will be measured: (1) adherence to a prescribed regimen of weekly treatment services for all clients, and (2) maintenance of a therapeutic milieu within the program. The first component will be assessed by applying innovative measurement models to daily activity log data for each participant. The second main element of fidelity, maintenance of a therapeutic milieu, will be measured using an instrument originally developed for use with adults in residential substance abuse treatment, but revised for the proposed study for use with an adolescent sample. Statistical process control procedures will then be used to analyze variability in the two main fidelity components for the program as a whole and for individual participants identified as outliers with respect to their fidelity scores. Finally, the relation between adherence to service provision parameters and ratings of therapeutic milieu will be examined.